1. Field of the Invention
The present invention relates to a self-cleaning method for a semiconductor exposure apparatus, and more particularly, it relates to a technology for optically cleaning a semiconductor exposure apparatus to remove a contaminant which adheres in the exposure apparatus.
2. Description of the Related Art
In manufacturing of a semiconductor integrated circuit, there is utilized a photolithographic technology in which an exposure apparatus is used to irradiate with light a photo-resist with which a semiconductor wafer is coated and to perform patterning, thereby forming a pattern of an element or a wiring line.
With an increase in integration density of a semiconductor circuit in recent years, there has been a need for forming a finer pattern having a higher density. As a solution for that, a shorter wavelength of an exposure light source has been used. Presently, an ArF excimer light source having a wavelength of 193 nm or an F2 light source having a wavelength of 157 nm is under development.
In an exposure apparatus using such a light source to form fine patterns having a high density, a contaminant in the air which adheres to a surface of an optical element causes harmful effects such as decrease and irregularity in illumination intensity. Therefore, nitrogen purge is carried out by which an optical path in the apparatus is purged with nitrogen gas to suppress the influence of the contaminants. However, even if such nitrogen purge is carried out sufficiently, there occurs such a harmful effect that the illumination intensity is deteriorated or made irregular due to contamination of the optical element when an optical path system is exposed to the air in maintenance or even in routine operation.
Therefore, especially in an exposure apparatus using an exposure light source having a short wavelength, a self-cleaning method referred to as optical cleaning is additionally used, by which ultraviolet light having a wavelength of 200 nm or less is emitted from a light source, and an optical system is irradiated with the ultraviolet light to cut off with strong energy of the ultraviolet light a chemical bond of a carbon compound, which is a main component of contaminants adhering to a surface of this optical system, thereby decomposing the compound.
For example, Jpn. Pat. Appln. KOKAI Publication No. 1993-335206 describes a technology, by which in a projection type exposure apparatus comprising an illumination optical system 1 for emitting illumination light, a filter 3 for passing through it only a wavelength useful to exposure by the illumination light, an exposure mask 4 on which a necessary exposure pattern is formed, a projection optical system 5 for projecting the exposure pattern to a subject 10 to be processed, a holder 6 to which the subject to be processed is mounted, and a stage 7 for moving the holder 6 with respect to the projection optical system 5. In its self-cleaning, an ultraviolet light filter 2 is placed in an optical path in place of the filter 3 and the exposure mask 4, so that the holder 6 is irradiated with ultraviolet light that has passed through this ultraviolet filter 2 through the projection optical system 5 in order to oxidize and remove organic foreign matters and impurities adhering to this holder 6.
However, a conventional projection type exposure apparatus has the following problems. That is, since the ultraviolet light filter 2 is placed on the optical path in place of the filter 3 and the exposure mask 4, the same positions along the same optical path as those in the case of ordinary exposure are irradiated with ultraviolet light for optical cleaning. Therefore, it is impossible to remove the contaminant adhering to a position other than the ordinary exposure path, so that the remaining contaminant might move to the exposure path and it might have a bad influence on the exposure path.